Generally, when completing a subterranean well for the production of fluids, minerals, or gases from underground reservoirs, a steel casing is placed into the wellbore and cemented into place. The casing provides for the ability to select zones in the wellbore to produce from. The sought after zones in the formation are accessed via explosively blasting a channel from the inside of the casing, through the casing, through the cement, and into the formation. Afterwards, subsequent completions operations are possible, including fracking, to stimulate and control the production of fluids from the formation.
Explosively perforating the formation using a shaped charge is a widely known method for completing an oil well. A shaped charge is a term of art for a device that when detonated generates a focused explosive output. This is achieved in part by the geometry of the explosive in conjunction with a liner. Typically, a shaped charge includes a metal case that contains an explosive material with a concave shape, which has a thin metallic liner. Many materials are used for the liner, some of the more common metals include brass, copper, and lead. When the explosive detonates the liner material is compressed into a super heated, super pressurized jet that can penetrate metal, concrete, and rock.
Shaped charges must be transported from a manufacturing facility to the field. The high explosives must be maintained and designed such that the risk of any premature detonation is mitigated against. Shaped charges are transported by a variety of transportation methods, in all climates and temperature ranges, and may be subject to temperature variations, vibrations, mishandling, and fire. They often have to travel across multiple legal boundaries, with varying degrees of safety requirements.
One of the safety requirements is that if the shape charge is set on fire, it will not detonate but instead will just burn or deflagrate. This requires that no pressure can build up inside of the shape charge, especially between the inner casing and the high explosive material while the explosive material is burning. Generally, obstructing materials such as retainer rings are not placed on the front face of the shape charge to hold all of the components in place as they could allow pressure to build up in the shape charge when it is deflagrating. A buildup in pressure while burning could lead to detonation of the shape charge.
Shaped charges contain many components that must be held into place effectively. Several methods for retaining the shape charge components will restrict the ability of the shape charge to vent gases in the event that the shape charge begins deflagrating due to a fire. In order to meet safety and transportation requirements, the shape charge must be designed such that if in the event the shape charge catches fire, the gases produced from the deflagration will safely vent out of the charge without substantial pressure buildup.